The present invention relates to electromotive devices including electromagnetically excited machines and permanent magnet synchronous machines (PMM""s). For clarity and simplicity, the term permanent magnet synchronous machine (PMM) is used throughout this specification, but it should be clearly understood that the term is intended to include electromotive devices in general, including electromagnetically excited machines.
The electromagnetic forces developed in a PMM result from the interaction of a permanent magnet field with a stator current. This interaction can be fundamentally represented by the relation:
F=k*B*K*S
Where:
F is the electromagnetic force
k is a proportionality constant depending on geometry,
B is the airgap flux density,
K is the armature surface current density, and
S is the airgap surface area.
In order to increase the power density of the PMM, it is necessary to increase the airgap flux density B or the armature surface current density K, or the airgap surface area S (however increasing the airgap surface area typically implies making a larger machine, which may well increase the power but not the power density of the device). The airgap flux density B is limited by the magnetic properties of the permanent magnets and the saturation properties of any flux-carrying components. The maximum current density K is ultimately limited by the conductor insulation temperature rating and the thermal demagnetization of the permanent magnets. These temperatures, in turn, depend on the machine""s inherent thermal characteristics and cooling mechanism. Most approaches to increasing power density use aggressive cooling methods to allow more current for a given temperature rise.
The present invention provides for electromotive device designs incorporating multiple phase windings, each winding including one or more notched ribbon conductors, which results in higher power density devices than those available in the prior art. The designs improve the performance of the parameters affecting the electromagnetic force generated by the device, especially the armature surface current density xe2x80x9cOKxe2x80x9d, and to some extent the airgap surface area S.
The notched ribbon conductors of the present invention may be utilized in axial gap machines, in radial gap machines, or in linear actuators, and may be applied to both coil and wave windings. The conductors may use a standard ribbon having a constant thickness and height, or they may use custom-shaped ribbon conductors which have variable thickness and/or variable height. In any event, the planar conductor or ribbon conductor has a thickness which is substantially less than the height (or width) of the conductor.
The windings of the present invention are typically divided into three areas: the working area (or working length) where the conductor cuts across the magnetic field generated by the magnets, the interior end-turn area, and the exterior end-turn area. The end-turns connect two working lengths of the winding, and they are typically also divided into two areas: the cross-over areas (or cross-over lengths, of which there are typically two at each end-turn and which include the area where two phase windings cross over each other), and the bridging area (or bridging length). The cross-over lengths may be further subdivided into transition lengths which flank the actual cross-over or intermesh area itself and thus provide a transition piece between the working length and the intermesh area, and between the intermesh area and the bridging length. Notches cut in the windings at the cross-over areas are cut so as to reduce the height dimension, at the notch area, of all the ribbon conductors comprising a winding.
The multiple phase windings of the present invention utilize ribbon conductors with notches in the phase cross-over areas instead of using wire conductors. This results in many advantages, including:
Lower electrical resistance: A ribbon winding has a higher copper fraction (i.e. % of copper, as opposed to insulation and air, which fills a slot area), with fractions of 90% or more possible in contrast to copper fractions in the 60% range for round wire windings. The windings of the present invention have notched crossover areas (areas where one phase winding crosses over another phase winding), and these notches increase the resistance to the flow of electricity. Thus, it is counter-intuitive to use notches in the cross-over areas. However, the use of these notches, as compared to bending of the windings past each other, results in a substantial reduction in the required length of the end-turns of the windings, and this reduction in end-turn length more than compensates for the increase in electrical resistance due to the notches. Note that the end-turn areas are typically, but not necessarily, non-work-producing areas of the windings, so reducing the length of these end-turns areas improves performance of the device by reducing the overall thermal and electrical resistances and by allowing a smaller device for a higher power density.
Lower thermal resistance: Thermal resistance is a property relating the temperature rise in the winding to its heat conduction (or heat flow). It is a measure of how difficult it is for heat to flow out of the winding. With single stator, dual rotor, axial gap PMM""s, heat generated by the windings primarily flows out through the exterior end-turn/housing interface. The high copper fraction of the ribbon windings of the present invention (almost equivalent to solid copper) allows heat to flow easily along the length of the winding to the end-turns, which are clamped to the PMM""s housing, which acts as a heat sink to cool the windings. Furthermore, the notched cross-over areas allow both axially oriented faces of the end-turns to contact the housing in the xe2x80x9cbridgingxe2x80x9d length of the end-turns, thereby increasing the thermal contact area for more effective cooling, since both end-turn faces are available for heat transfer.
Greater Magnetic Airgap area: The use of notches resulting in planar stator faces often allows the magnetic airgap surface area S to be increased by extending partially over the end turn areas, where the conductors are transitioning from radial to tangential orientations, but still have a significant radial component which can contribute to torque production.
More Compact Structure: The use of notches allows the radial height of the end-turns (as well as lengths) to be shorter for a more compact winding structure. This, in turn, allows smaller machines with greater power density to be constructed.
Another benefit of this PMM design is that the actuator housing can be totally enclosed with minimal impact of cooling performance (since cooling primarily occurs by end-turn conduction, not internal convection of the windings).